Genetically modified organisms in agriculture

Biology A2 · Genetic technology. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

The human population is rising, so more food is needed from the same land and water. Genetic engineering can raise the productivity or the quality of farmed animals and crop plants faster than selective breeding can.

You need three examples. GM salmon have an added growth hormone gene and grow to market size in about half the usual time. Herbicide-resistant soybean survives a weedkiller that kills the weeds around it. Insect-resistant cotton makes its own toxin that kills the insect larvae feeding on it.

You must also be able to discuss both sides: higher yields and less spraying against risks to the environment, costs to farmers and public worry.

Rules to remember

GM salmon: a growth hormone gene with a promoter from another fish, active all year, so growth hormone is made continuously.
GM salmon are kept as sterile females in tanks on land so the gene cannot reach wild salmon.
Herbicide-resistant soybean: the field can be sprayed with glyphosate; the weeds die, there is less competition and the yield rises.
Bt cotton has a gene from the bacterium Bacillus thuringiensis for a protein toxic to insect larvae, so less insecticide is sprayed.
Environmental concerns: pollen may carry genes to wild relatives; pests and weeds may evolve resistance; fewer weeds means less food for wildlife.
Social and economic concerns: seed is patented and costly and must often be bought each year; people want labelling and a choice.
Benefits: more food from the same area of land, so less natural habitat needs to be cleared.

Common mistake

Students write that Bt cotton is resistant to herbicide or that the plant is sprayed with Bt toxin. Bt cotton makes the insect toxin in its own cells; herbicide resistance is the soybean example.

Worked example

Farmers growing Bt cotton are often told to plant a strip of non-GM cotton beside it. How does this slow the evolution of pests that are resistant to Bt toxin?

In a field of Bt cotton only the rare resistant insects survive, so selection for resistance is very strong.
On the non-GM strip, insects without resistance survive in large numbers.
These mate with the few resistant insects from the Bt crop, so resistance alleles stay at a low frequency in the population.

Answer: The non-GM strip keeps many non-resistant insects in the population, so resistance alleles spread more slowly.

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